The present invention relates to systems for monitoring changes in shape of a structure.
A jig is a framework for holding and aligning parts being assembled. A large jig used to assemble a large structure, such as an aircraft body, tends to change shape due to changes in the foundation on which the jig rests, in ambient temperature, and in loads carried. Since changes in shape affect the alignment of parts being held, a large jig is suitably fitted with mechanisms which enable operators to adjust jig shape. However, in order to properly adjust the shape of a jig, it is necessary to provide a means for accurately monitoring jig shape.
Jig shape can be ascertained by monitoring the alignment of a set of points on the jig with respect to one or more reference lines. The points to be monitored are selected in such a way that the amount and direction of deviation of a point from proper alignment provides an indication of how to adjust one or more jig adjustment mechanisms. A number of systems have been developed to monitor the alignment of selected points on a structure with respect to lines defined by the paths of laser beams. A typical system of the prior art is described in U.S. Pat. No. 3,603,691, issued Sept. 7, 1971 to Ralph A. Hamilton, wherein laser beams are directed at photodetectors mounted at various points of interest on the jig. As the jig changes shape, the photodetectors move with respect to the laser beams and the portion of a beam striking each photodetector changes, thereby affecting the magnitude of the output signal of each photodetector. Hamilton utilizes a set of four photodetectors at each point on the jig where two-dimensional motion of the jig in a plane orthogonal to the laser beam is of interest. The centroid of the beam along each of two orthogonal axes in the plane is determined by linear interpolation of relative magnitudes of the output signals produced by a pair of photodetectors spaced along each axis. Hamilton uses three laser beams to monitor changes in jig shape with six degrees of freedom. All three laser beams are aligned in parallel with two beams contained within a common vertical plane and the third beam being contained within another vertical plane.
Hamilton indicates the system is capable of detecting displacements of 0.005 inches at distances of 200 feet. However, in assembly of large aircraft bodies the ability to detect displacements an order of magnitude smaller would be beneficial. The accuracy and resolution with which Hamilton's system can measure jig alignment is limited by the accuracy and resolution of the linear interpolation method used to locate the centroid of the beam as it strikes the photodetectors. The cross-sectional intensity distribution of a laser beam in the plane orthogonal to the beam is usually a non-linear Gaussian distribution, and determination of the centroid of the beam by linear interpolation of the magnitudes of photodetector output signals can be inaccurate. In addition, the direction of a laser beam tends to drift somewhat over time due to temperature change in the laser apparatus, and due to bending, movement or compression of the platform upon which the laser source is mounted or fluctuations in air density. A drifting laser beam delivers a false impression of jig movement, particularly at points remote from the laser source.